Hydrocarbon charge history of the Paleogene reservoir in the northern Dongpu Depression,Bohai Bay Basin,China

The hydrocarbon charge history of the Paleogene in the northern Dongpu Depression was analyzed in detail based on a comprehensive analysis of the generation and expulsion history of the major hydrocarbon source rocks, fluorescence microscopic features and fluid inclusion petrography. There were two main stages of hydrocarbon generation and expulsion of oil from the major hydrocarbon source rocks. The first stage was the main hydrocarbon expulsion stage. The fluorescence microscopic features also indicated two stages of hydrocarbon accumulation. Carbonaceous bitumen, asphaltene bitumen and colloidal bitumen reflected an early hydrocarbon charge, whereas the oil bitumen reflected a second hydrocarbon charge. Hydrocarbon inclusions also indicate two distinct charges according to the diagenetic evolution sequence, inclusion petrography features combined with the homogenization temperature and reservoir burial history analysis. According to these comprehensive analysis results, the hydrocarbon charge history of the Paleogene reservoir in the northern Dongpu Depression was divided into two phases. The first phase was from the late Dongying depositional period of the Oligocene to the early uplift stages of the late Paleogene. The second phase was from the late Minghuazhen period of the Pliocene to the Quaternary. Reservoirs formed during the first period were widely distributed covering the entire area. In contrast,reservoirs formed during the second period were mainly distributed near the hydrocarbon generation sags. Vertically, it was characterized by a single phase in the upper layers and two phases in the lower layers of the Paleogene.

Determination of Hydrocarbon Charging History by Diagenetic Sequence and Fluid Inclusions:A Case Study of the Kongquehe Area in the Tarim Basin

Petrographic analysis of hydrocarbon inclusions in reservoirs is the basis and prerequisite for study of hydrocarbon charge history using fluid inclusion analysis.Samples from Silurian reservoirs in the Kongquehe area were studied with microscopy,cathode luminescence and scanning electron microscopy,and the paragenetic sequence of diagenetic events was established.Aqueous and oil inclusions were found in four different occurrences,i.e.,① in healed cracks in detrital quartz grains,② in quartz overgrowths that were formed relatively early in diagenesis,③ in healed cracks crosscutting quartz overgrowths and detrital quartz,and ④ in paragenetically late calcite cements.Solid bitumens were found in intergranular pores and in late fractures,whereas gas inclusions occur in healed cracks crosscutting quartz overgrowths and detrital quartz.The homogenization temperatures of aqueous（Th_（aq）） and oil incluisons（Th_0） within individual fluid inclusion assemblages are very consistent,suggesting that the microthermometric data are reliable.The Th_（aq） values are generally larger than Th_0,indicating the oil charging events took place at significant depths.The results suggest that there were at least two episodes of hydrocarbon charging in the Kongquehe area：the early hydrocarbon charging occurred in late Caledonian,dominated by oil,and the late hydrocarbon charging occurred in the Yanshan-Himalayan,first by oil and then gases.In addition,two episodes of hydrocarbon reservoir adjustment and destruction occurred in the Hercynian and Himalayan,respectively,forming solid bitumen.

Key Elements Controlling Oil Accumulation within the Tight Sandstones

Tight oil sandstone reservoirs with low porosity and permeability, which are an unconventional petroleum resource, have been discovered in the Jurassic intervals of the central Junggar Basin, the northwestern China. To reveal the accumulation mechanism, a relatively comprehensive research was conducted, including oil-source correlation, porosity evolution, and hydrocarbon charging history. The results show that crude oil of these tight sandstone reservoirs were mainly from Permian source rocks with some contribution from Jurassic source rocks. The reservoirs were buried at shallow depth（〈3 100 m） and exposed to weak diagenesis, and thus had high porosity（18.5%） when the Permian-sourced oil from Permian source rock was charging, indicating high GOI values（〉5%）. In contrast, the sandstone reservoir had already become tight and did not provide available space to accumulate oil due to severe compaction and cementation when hydrocarbon from Jurassic source rock filled, evidenced by low GOI values（〈5%）. Therefore, reservoir porosity controls the oil accumulation within tight sandstone. Whether tight sandstone reservoirs accumulate oil depends on the reservoir quality when hydrocarbons charge. Before the exploration of tight oil sandstone reservoirs, it should be required to investigate the relationship between oil charging history and porosity evolution to reduce the exploration risk and figure out the available targets.